This invention relates generally to cathode ray tubes (CRTs) of the beam index-type and is particularly directed to a multi-beam index CRT having horizontal phosphor bands.
One common cathode ray tube (CRT) employs a color selection electrode in the form of a thin apertured sheet commonly known as a xe2x80x9cshadow maskxe2x80x9d. The shadow mask is in closely spaced relation to an inner surface of the CRT""s glass faceplate which has electron beam sensitive phosphor either in the form of bands or dots disposed thereon. The three electron beams are typically directed through apertures in the shadow mask onto the phosphor screen for emitting the primary colors of red, green and blue which appear in the form of a video image on the faceplate. The apertures in the shadow mask ensure that each beam lands only on its associated color phosphor element to provide a high degree of color purity in the video image. Even with precise alignment between the electron guns, shadow mask and phosphor elements on the display screen, a substantial portion of each electron beam is intercepted by the shadow mask prior to incidents upon the faceplate. For example, the shadow mask typically intercepts and dissipates 80% of the electron beam before it reaches the phosphor screen. This not only limits video image brightness, but also results in heating and expanding of the shadow mask and causes misalignment between the shadow mask apertures and electron beam positions which reduces color purity.
Another approach to CRT design is known as a beam index CRT which eliminates the shadow mask. In a beam index CRT, an electron beam is deflected over phosphor bands or stripes disposed on the inner surface of the CRT""s faceplate. The parallel, linear phosphor bands are typically oriented vertically and disposed across the CRT""s faceplate in a horizontal direction, which is the same direction as electron beam movement. A sensor in the funnel region provides an index signal whose timing is indicative of the position of the CRT""s electron beam relative to the various phosphor bands on the faceplate. Because the index signal is a function of the position of the electron beam relative to the phosphor bands, it is used to control the selection of the input drive signals to the CRT""s electron gun for providing a video image component at a predetermined location on the faceplate in accordance with the received video signal.
With the electron beam deflected horizontally across the faceplate and with the phosphor bands oriented generally vertically and disposed in a spaced manner across the faceplate, the electron gun must be turned on and off at precisely the right instant and at a very high frequency. For example, with a horizontal sweep time of 62.4 microseconds and with 400 color pixels for horizontal scan line, or 3xc3x97400=1,200 monochrome pixels per line, the electron beam dwell time at each pixel is on the order of 52 nanoseconds. This requires a flat frequency response of almost 100 MHz which is difficult to achieve.
In addition, because the electron beam cannot be instantaneously turned off or on, the beam distribution on a given vertical phosphor band is gaussian. This results in a portion of the electron beam being incident upon portions of the CRT faceplate between adjacent vertical phosphor bands which is nonemissive and results in reduced video image brightness.
Another approach in beam index CRT design employs horizontally aligned phosphor elements arranged in alternating red, green and blue color producing stripes. A single electron beam or three electron beams may be provided for energizing the respective red, green and blue phosphor stripes. To provide satisfactory video image resolution, a large number of thin phosphor stripes must be employed. In a beam index CRT incorporating horizontal phosphor stripes, the vertical position registration of the electron beam must be maintained to within a few mils of its proper position which is centered on the particular phosphor stripe being scanned. An electron beam sensing and feedback control arrangement is typically employed for aligning the electron beam with the phosphor stripe it is scanning. The vertical spacing between adjacent electron beams limits the color convergence of the electron beams which typically require a relatively sophisticated main lens arrangement for converging and focusing the electron beams on the display screen. The use of a single electron beam eliminates the multi-beam convergence problem, but requires a large current in the single electron beam, and three times faster scan rate to cover the three individual color fields.
The present invention overcomes the aforementioned limitations of the prior art by providing a beam index CRT having a plurality of spaced, vertically offset electron beams each adapted to scan a respective horizontally aligned phosphor stripe on the display screen for providing one of the primary colors of a video image. Each of the electron beams is horizontally elongated in cross section, with the scanning beams aligned with the horizontal phosphor stripes by means of an auxiliary deflection coil and beam vertical position feedback control loop combination and with beam color convergence provided by a plurality of adjustable multi-pole magnets.
Accordingly, it is an object of the present invention to provide an improved CRT of the beam index type.
It is another object of the present invention to provide a beam index CRT having a multi-beam electron gun with vertically and horizontally spaced electron beams for simultaneously providing color video information on adjacent, vertically spaced, horizontal scan lines.
A still further object of the present invention is to eliminate the requirement for high frequency ON/OFF cycling of an electron beam in a vertical stripe beam index type of CRT.
Yet another object of the present invention is to provide improved electron beam convergence in a multi-beam index-type color CRT using an open main lens incorporating cylindrical focusing grids.
The present invention contemplates a beam index cathode ray tube (CRT) comprising a display screen having a plurality of vertically spaced, horizontally aligned, parallel linear phosphor stripes disposed on an inner surface thereof; an electron gun including cathode means for providing energetic electrons; a beam forming region (BFR) for forming the energetic electrons into a plurality of spaced electron beams each having a horizontally elongated cross section, wherein one or more of the beams are vertically offset from one another; a high voltage focusing lens disposed intermediate the BFR and the display screen for focusing the electron beams on the display screen in the form of a plurality of vertically offset electron beam spots each disposed on a respective phosphor stripe; and an electromagnetic deflection arrangement disposed intermediate the electron gun and the display screen for deflecting the electron beams over the display screen in a raster pattern, wherein each electron beam is incident upon and each electron beam spot scans a respective phosphor stripe.